For guiding laser beams, it is customary to use either light conductors in the form of flexible glass fibers or transmission systems containing optical deflectors such as mirrors. Particularly in medical applications such as laser surgery, for example, articulated transmission systems of the last-mentioned type are frequently used because so far there are no fiber cables which are sufficiently resistant to rupture while at the same time providing for a sufficiently low-loss transmission in the wavelength range of the carbon-dioxide lasers predominantly used in these applications.
The articulated arms used for guiding the laser beam include a series of deflection mirrors secured to rotatable joints such that each mirror is rotatable about the axis of the incident laser beam together with the follow-on part of the pivot arm. In addition, telescope guides are often inserted between the joints thereby enabling the length of the arm to be changed. Articulated arms having as many as seven rotatable joints are known.
Apart from being dependent on a sufficiently precise adjustment of the mirrors, the proper function of such an articulated arm depends on the quality of the pivot bearings and telescopic guides as well as on the rigidity of their connections. The problems associated therewith become greater as the dimensions of the work area covered by the articulated arm increase. Adding more rigidity to the arm requires stronger material and, accordingly, larger masses to be moved. This puts an increased load on the pivot bearings and impairs handling, particularly in dynamic operation.
If, however, guidance errors occur, the beam will wander from its predetermined beam path. First of all, this will cause the laser focus to shift which is not particularly disturbing within certain limits, especially if the focus position is visually evaluated by means of a reflected pilot beam. Where long articulated arms are used, the situation can occur that the laser beam within the articulated optical system wanders out of the free openings of the deflection mirrors or the focusing optics and impinges on their mountings. In this event, high-performance lasers will destroy the optical system.
In laser systems for reading out the contents of information storage plates, it is known to provide control arrangements which make the focus of the laser beam follow a predetermined track radially. These control arrangements, however, evaluate the deviation of the focus from the visibly marked data track. In addition, there exists a fixed spatial relationship between the direction of movement of the positioning member and that of the laser focus. However, if the laser beam is guided via several articulated joints, the angle of rotation of each individual joint has to be taken into consideration when determining the relationship between the position coordinates of the focus and the positioning coordinates of the member performing the follow-up function. This is not easily possible since the complexity of the arrangement would increase considerably if each articulated joint were provided with an angle sensor of its own, particularly in articulated arms including several pivot axes.
Japanese patent publication Nos. JP-A-57-4393, JP-A-58-224088 and JP-A-57-154389 disclose regulating devices for laser machining tools by means of which the laser beam is prevented from wandering out of the axis of the optics guiding the beam. A position-sensitive detector disposed in the vicinity of the exit of the optical system is provided for recognizing the beam position, and a portion of the laser radiation is aimed at this detector. A positioning mirror preceding the optical system on the entry side is acted upon by the detector signal.
In these devices, the transmission path for the laser beam is fixed, so here also there is a fixed spatial association between the direction of movement of the positioning member and the deflection of the laser beam at the exit end. The known devices are not suitable for regulating the beam position of a laser beam guided via an articulated optical system capable of rotational movement. In U.S. Pat. No. 4,659,916 an apparatus for positional correction of a laser beam guided via an articulated optical system is described in which the direction of the laser beam is modulated by a swinging mirror disposed on the entry end. A detector at the exit of the articulated optical system provides a signal at the modulation frequency which enables the recognition of the rotational position of the articulated optical system that is required for regulation to be performed. Here, however, electrical supply lines leading to the movable end of the articulated optical system are necessary. The apparatus is also expensive, because of the parts required for modulation and demodulation of the beam direction.